Methods for assaying ferrous ion in aqueous liquids such as biological liquids (blood, serum and urine) are known. Such assays are carried out on human serum for the purpose of diagnosing conditions such as anemia, liver disorders and lead poisoning. It is also of interest, for example, to analyze the ferrous ion content in boiler water in steam electric power generating plants. Known methods for ferrous ion determination are generally referred to as wet methods. The reagents involved are first dissolved or suspended in a liquid aqueous vehicle. The resulting aqueous reagents are mixed together and then the ferrous ion measurement is made colorimetrically.
None of the known wet methods for ferrous ion determination have been adapted for use in so-called dry methods. Dry chemical methods have reference to chemical methods which are performed using reagent compositions incorporated in various substantially "dry-to-the-touch" elements. Examples of such elements include "dip and read" test strips and multi-zone analytical test elements. The latter elements are disclosed for example, in U.S. Pat. No. 4,132,528. Also, some of the most effective wet methods for ferrous ion determination are not adaptable to dry chemical techniques because of insufficient sensitivity due to the low extinction coefficients of the ferrous ion-dye complex developed by the method; (2) lack of selectivity from interfering metal cations such as cupric (Cu.sup.+2) and zinc (Zn.sup.+2) cations and (3) insufficient rate of metallization at the usual operating pH's of about 4 to 5 necessary to dissociate ferric ion from transferrin in serum assays.